Industrial processes frequently require raising the temperature of large numbers of billets of metal in a continuing operation. The usual billet furnace is essentially a long passageway heated to high temperatures, and traversed by a conveyor. The billets are usually round blocks of metal of essentially the same size, and move down the conveyor in closely-spaced relationship as the temperature is increased to the desired level. Heat is usually supplied by a series of torch-like burners spaced along the passageway closely enough so that temperature variation along the path traveled by the billets can be held close to the desired level or gradient. Since the object of these furnaces is to produce perdetermined temperature conditions within the billets, it has become standard practice to check and control the operation of the furnace by thermocouple probes that are periodically projected into the furnace to engage a particular billet to sense its temperature. Installations of this type are common in connection with aluminum extrusion processes, where it is necessary to raise the billets to a temperature sufficient to allow the extrusion, without the raising the plasticity of the billets to the point where they cannot be handled by the associated automated equipment. It is also obvious that the temperature-sensing function has the effect of preserving the integrity of the furnace itself, as an excess in temperature will usually cause the damage to the lining, housing, or conveyor of the furnace.
A common maintenance problem is encountered in connection with the probe rods. Repeated exposure to the high temperatures of the furnace can produce oxidation of the tips of the probes, which injects an unknown variable to the electrical resistence. Since the probes are usually shoved into the billet hard enough to penetrate the surface slightly, repeated cycles of this action also tend to dull the sharpened points of the probe. A thermocouple produces a small electrical sensing signal as a function of the effect of temperature on probe rods of different material; and minor variations of the resistence at the point of contact will have a serious effect on the magnitude of this signal, resulting in serious problems as the signal is used for the control of the furnace temperature. Maintenance of acceptable conditions at the points of the probes requires repeated sharpening of the probe tips, and ultimately a replacement of the entire probe rods after a relatively short decrease in its length. The conventional practice requires sufficient disassembly of the probe system to get at the rods, followed by either re-grinding the tips, or by the replacement of the entire rods.
The usual arrangement for providing access to the interior of the furnace for the probe assembly is to surround a trap door in the furnace structure with a box-like chamber that can be pressurized to match or coordinate with the pressure conditions within the furnace. The box provides an opening large enough to accommodate the outer cantilever housing of the probe assembly, which is extended through this opening into the chamber, and then through the trap door into the furnace to engage one of the billets. Protection of the equipment associated with the probe assembly requires that the escape of hot gases around the probe assembly housing be kept to a minimum, but practical considerations limit the closeness of the fit that it is possible to maintain at this point. The problems outlined above have generated a maintenance nuisance that is capable of seriously interferring with the operation of the entire system.